Many computers now employ a mezzanine bus that allows high speed communication between the processor, memory, and peripheral devices. These computers generally also provide bridges to other, often older, types of buses. Providing these bridges can allow the processor to communicate efficiently with devices employing specialized interfaces, with legacy devices that use older bus interfaces, and with slower or less expensive devices that do not need to communicate at high speeds.
The bridges provided on existing computers do not always use consistent access methods, even when two or more are connected to the same bus. For example, bridges between a PC Interactive (PCI) mezzanine bus and Industry Standard Architecture (ISA) buses can operate in either positive or subtractive modes. In positive mode, the ISA address space is allocated as a portion of the PCI address space, while in subtractive mode, those requests that are not claimed by the PCI bus can be claimed by the ISA bus.